A magnetic random access memory (MRAM, hereinafter referred to as “MRAM”) is a non-volatile random access memory technology using magnetic elements. By way of example, a spin transfer torque magnetic random access memory (STT-MRAM, hereinafter referred to as “STT-MRAM”) uses electrons that become spin-polarized when passing through a thin film (spin filter). The STT-MRAM is also known as a spin transfer torque RAM (STT-RAM), a spin torque transfer magnetization switching RAM (spin-RAM), and a spin momentum transfer RAM (SMT-RAM).
The MRAM is a first generation magnetic memory as a non-volatile random access memory that is capable of reading and writing data, rewriting data unlimited times, and semipermanently storing data at a similar level of a SRAM. However, the MRAM has a small memory capacity and a high power consumption as compared with a DRAM.
Meanwhile, as a new generation magnetic memory, the STT-MRAM avoids a problem of the MRAM with a small memory capacity and is expected to have a memory capacity equivalent to that of the DRAM. The STT-MRAM changes a magnetization direction of a magnetic body by using a magnetic moment generated when electrons are spun and is suitable for miniaturization and high densification.
In a STT-MRAM cell, when a datum “0” is written to a magnetic tunnel junction device, a high voltage is applied to a bit line, and when a datum “1” is written to the magnetic tunnel junction device, a high voltage is applied to a source line. That is, unlike other memory structures, the source line is not grounded (Vss) all the time, a certain level of voltage may be applied to the source line.
FIG. 1A illustrates a structure of a conventional magnetic random access memory. A STT-MRAM includes a transistor, a magnetic tunnel junction device, a gate line, a bit line, and a source line. The magnetic tunnel junction device is comprised of a stationary layer, a magnetization layer, and a thin insulating layer. The gate line and the bit line are arranged so as to be perpendicular to each other. The source line is arranged so as to be typically parallel to the bit line or perpendicular to the bit line depending on a specific architecture used in the STT-MRAM. The bit line is connected to the magnetic tunnel junction device, and the source line is connected to a source of a substrate.
FIGS. 1B and 1C illustrate a cell array of the conventional magnetic random access memory. In a magnetic random access memory cell depicted in FIGS. 1B and 1C, if the source line is arranged so as to be parallel to the gate line, an area of a unit cell 10 can be minimized to 8 F2. However, if the source line is operated with a high voltage in order to write a datum “1”, the gate line parallel to the source line is also operated. Thus, a parasitic capacitance to be applied to the source line is greatly increased. Therefore, an operation speed is greatly reduced.
FIGS. 2A and 2B illustrate another cell array of the conventional magnetic random access memory. In order to avoid a problem of the structure depicted in FIG. 1C, FIGS. 2A and 2B use a method in which the source line is arranged so as to be perpendicular to the gate line in the magnetic random access memory to reduce a parasitic capacitance to be applied to the source line. However, in this method, the source line needs to be arranged to be parallel with the bit line while avoiding a short circuit at a contact between the magnetic tunnel junction device and an active layer. Thus, an area of a unit cell 20 can be increased to 12 F2.
In this regard, Korean Patent Laid-open Publication No. 10-2004-0111716 (entitled “Dense array structure for non-volatile semiconductor memories”) describes a semiconductor memory device having an array arrangement of overlapped word lines and diagonal bit lines in a column direction and a row direction. Further, Korean Patent No. 10-0536592 (entitled “Magnetic memory and method for fabricating the same”) describes a magnetic memory including a magnetic tunnel junction device and a method for arranging a bit line and a digit line to be intersected with each other.